CVE-2026-53122

In the Linux kernel, the following vulnerability has been resolved:

btrfs: fix deadlock between reflink and transaction commit when using flushoncommit

When using the flushoncommit mount option, we can have a deadlock between a transaction commit and a reflink operation that copied an inline extent to an offset beyond the current i_size of the destination node.

The deadlock happens like this:

1) Task A clones an inline extent from inode X to an offset of inode Y that is beyond Y's current isize. This means we copied the inline extent's data to a folio of inode Y that is beyond its EOF, using a call to copyinlinetopage();

2) Task B starts a transaction commit and calls btrfsstartdelalloc_flush() to flush delalloc;

3) The delalloc flushing sees the new dirty folio of inode Y and when it attempts to flush it, it ends up at extentwritepage() and sees that the offset of the folio is beyond the isize of inode Y, so it attempts to invalidate the folio by calling folioinvalidate(), which ends up at btrfs' folio invalidate callback - btrfsinvalidate_folio(). There it tries to lock the folio's range in inode Y's extent io tree, but it blocks since it's currently locked by task A - during a reflink we lock the inodes and the source and destination ranges after flushing all delalloc and waiting for ordered extent completion - after that we don't expect to have dirty folios in the ranges, the exception is if we have to copy an inline extent's data (because the destination offset is not zero);

4) Task A then attempts to start a transaction to update the inode item, and then it's blocked since the current transaction is in the TRANSSTATECOMMITSTART state. Therefore task A has to wait for the current transaction to become unblocked (its state >= TRANSSTATE_UNBLOCKED).

So task A is waiting for the transaction commit done by task B, and the later waiting on the extent lock of inode Y that is currently held by task A.

Syzbot recently reported this with the following stack traces:

INFO: task kworker/u8:7:1053 blocked for more than 143 seconds. Not tainted syzkaller #0 "echo 0 > /proc/sys/kernel/hungtasktimeoutsecs" disables this message. task:kworker/u8:7 state:D stack:23520 pid:1053 tgid:1053 ppid:2 taskflags:0x4208060 flags:0x00080000 Workqueue: writeback wbworkfn (flush-btrfs-46) Call Trace: <TASK> contextswitch kernel/sched/core.c:5298 [inline] __schedule+0x1553/0x5240 kernel/sched/core.c:6911 __scheduleloop kernel/sched/core.c:6993 [inline] schedule+0x164/0x360 kernel/sched/core.c:7008 waitextentbit fs/btrfs/extent-io-tree.c:811 [inline] btrfslockextentbits+0x59c/0x700 fs/btrfs/extent-io-tree.c:1914 btrfslockextent fs/btrfs/extent-io-tree.h:152 [inline] btrfsinvalidatefolio+0x43d/0xc40 fs/btrfs/inode.c:7704 extentwritepage fs/btrfs/extentio.c:1852 [inline] extentwritecachepages fs/btrfs/extentio.c:2580 [inline] btrfswritepages+0x12ff/0x2440 fs/btrfs/extentio.c:2713 do_writepages+0x32e/0x550 mm/page-writeback.c:2554 __writebacksingleinode+0x133/0x11a0 fs/fs-writeback.c:1750 writebacksbinodes+0x995/0x19d0 fs/fs-writeback.c:2042 wbwriteback+0x456/0xb70 fs/fs-writeback.c:2227 wbdowriteback fs/fs-writeback.c:2374 [inline] wbworkfn+0x41a/0xf60 fs/fs-writeback.c:2414 processonework kernel/workqueue.c:3276 [inline] processscheduledworks+0xb6e/0x18c0 kernel/workqueue.c:3359 workerthread+0xa53/0xfc0 kernel/workqueue.c:3440 kthread+0x388/0x470 kernel/kthread.c:436 retfromfork+0x51e/0xb90 arch/x86/kernel/process.c:158 retfromforkasm+0x1a/0x30 arch/x86/entry/entry64.S:245 </TASK> INFO: task syz.4.64:6910 blocked for more than 143 seconds. Not tainted syzkaller #0 "echo 0 > /proc/sys/kernel/hungtasktimeoutsecs" disables this message. task:syz.4.64 state:D stack:22752 pid:6910 tgid: ---truncated---